Plant Science for August 12, 1999

Planting date influences corn hybrid maturity and the thermal (or heat units needed to grow) responses
of the crop during the season. In a study run in 1993 at four locations through Indiana and Ohio,
researchers determined that delayed planting shortens the thermal intervals for corn throughout the season.

In other words, delayed planting decreased the time between planting and silk emergence, especially
at the more northern locations. In fact, the growing degree days (GDDs) to mid-silk were decreased as
much as 126 GDDs for mid-June plantings as compared to early May plantings. One way corn adapted
to a late planting in order to compensate for the shorter season was to slightly decrease the final leaf number
on plants in the mid-June planting for four of the five hybrids planted.

Delayed planting also decreased the thermal interval between mid-silk and kernel black layer development,
especially in northern locations. And, as expected, the later maturity hybrids were more affected than earlier
hybrids. On the average, the GDD change from mid-silk to black layer was decreased from 4.5 to 7.4 GDD
per day of delayed planting among the hybrids used in the study. Thus, season-long effects can be seen from
delayed planting of corn and can affect more northern locations substantially.

Averaged over three years of a similar study, delayed planting was shown to decrease the thermal time
from planting to kernel black layer by 4 to 9 GDD per day of delayed planting for the three hybrids that
were evaluated during all three years. Growth and development of corn has been known to be closely
related to GDDs accumulated over calendar time. And, the "short-season" hybrids typically require fewer
GDDs to reach silking and black layer than do the "fuller-season" varieties.

Many seed companies provide days to maturity ratings on the hybrids sold and often use the GDD ratings
for the product as a secondary descriptor. Unfortunately, a standard GDD-based maturity rating scheme is
not used in the seed industry. The differences are from different GDD calculation methods, initiating GDD
accumulation at different times (date of seedling emergence rather than from the planting date), differences
in defining the actual date of black layer formation or with genotype by environment interactions that vary
the relation between GDD accumulation and growth stage development.

To complicate matters, GDD accumulations required for corn germination and emergence vary among
locations and years, often due to the variability of soil temperatures and photoperiod. Once emerged, however,
the vegetative growth rate on corn is fairly consistent with GDD accumulation as long as plant stress is not
present. This parallel is not as clear between silking and black layer formation and can even be varied
depending on hybrid grown. Some hybrids can modify their GDDs requirements a bit, while other hybrids
are less able to compensate.

In fact, the limited compensation ability in corn may be due to daylength or hybrid photoperiod sensitivity.
In a 1983 study, it was determined that corn sensitivity to photoperiod begins 4 to 8 days prior to tassel
initiation and continues until shortly after tassel initiation. However, later studies in 1986 modeled corn growth
rate under a range of daylength variations prior to tassel initiation and still found that hybrids adapted differently
based on early photoperiod changes. However, to confirm both studies, the effects (timing requirement) of
delayed planting on hybrids from emergence to silking were relatively similar across hybrids tested but was an
even greater linear decrease in thermal interval and with greater differences among hybrids for the effects of
delayed planting from silking to black layer formation. In other words, more of the corn compensation ability
is seen during the silking to black layer formation development period. And, more "grain fill time" is lost with
delayed or late planting than is with a more tailored and early planting date. This does seem to indicate that
corn "monitors" the daylength and "adapts" based on this monitoring as well as on any plant stresses that the
corn plant may be experiencing.

Besides hastening development (requiring less GDDs), delayed planting primarily is seen as affecting
grain fill, ultimately yield. The time line between silking and black layer formation in corn is when the hybrid
is kernel filling from initiation at pollination to blister to milk to dough to dent to finishing grain fill at
physiological maturity (or black layer). In the three year study, the interval from silking to milk line development
was the time interval when the GDD requirement was decreased the most per day of delayed planting,
emphasizing again the importance of the time period during pollination and beyond for corn yield gain as
well as the need to plant in a timely manner.

Other studies have repeated the above information. And, while corn can't "compensate" quite as well as
soybeans in cutting back on GDDs requirements toward physiological maturity, corn has been found to
frequently adapt by lessening the time required by 3 to 4 days (rarely more than a week) during poor seasons
or with delayed planting.

Now is a good time to look at your hybrid maturity rating and determine where you stand in the season
against the average date of the first fall frost. Commonly the GDDs required for corn in North Dakota and
the northern growing regions of Minnesota are as follows:

GDDs	Relative Maturity (days)
1750-1850	70
1851-1950	75
1951-2050	80
2051-2150	85
2151-2250	90
2251-2350	95
2351-2450	100
2451-2500	105

The average first fall frost (first 32 F date) varies through the region from September 16 to September 25,
going from the south end of the Valley up to the Canada border. Use the NDAWN (North Dakota Agricultural
Weather Network) on the internet at:

to access the program for GDD determination for corn to more accurately pinpoint the amount of GDDs
accumulated from your location using your date of planting in order to determine the approximate number
of GDDs you need for corn to reach physiological maturity. GDDs from this point to September 15 range
each day from 5 to 15. Figure a minimum number and a maximum number of remaining GDDs for your area
based on the range or based on the normals seen in other years in your location to determine how your corn
crop is progressing for the season and if your corn is on target for good yields.

While global positioning systems (GPS) have been slowly improving for agricultural uses through the use of
the system with software to record and map crop fields, the layering information just got better. Current
technology has now linked GPS and a video camera that allows you to bring photos and movies from the field
back to your office.

While this might mean the purchase of a new software (and a more powerful computer) package in the
future, consider the unlimited possibilities with the advancement. Video of field conditions will allow you
to optimize treatment timings and have a recording of field management actions and resulting outcomes. The
video linkage will allow crop specialists to view the field from a distance so that "scouting" timing can be
optimized. Problems in the field, such as herbicide injury or resistant weed problems, can be captured digitally
and used to diagnose the problem.

Integrating the link of GPS to video will allow you to review the management from a previous year that
was similar in environment to help you determine what to do in a wet year versus a dry year, an early spring
versus a late planting and an early fall harvest versus an extended fall.

The small hand-held video cameras used with the system are linked to a very small and portable digital
receiving system which can then be sent or directly compiled through your computer system, complete with
GPS mapping specifics. Already recorded variety, field, and other crop input data and timings will allow the
ease of better management with this additional information.